1. Field of the Invention
The present invention relates to a movable belt that may be used in a belt separation apparatus to separate a particle mixture based on charging of the particles, and more specifically to an improved belt and a method of belt construction.
2. Discussion of Related Art
Belt separator systems (BSS) are used to separate the constituents of particle mixtures based on the charging of the different constituents by surface contact (i.e. the triboelectric effect). FIG. 1 shows a belt separator system 10 such as is disclosed in commonly-owned U.S. Pat. Nos. 4,839,032 and 4,874,507, which are hereby incorporated by reference in their entirety. One embodiment of belt separator system 10 includes parallel spaced electrodes 12 and 14/16 arranged in a longitudinal direction to define a longitudinal centerline 18, and a belt 20 traveling in the longitudinal direction between the spaced electrodes, parallel to the longitudinal centerline. The belt 20 forms a continuous loop which is driven by a pair of end rollers 22, 24. A particle mixture is loaded onto the belt 20 at a feed area 26 between electrodes 14 and 16. Belt 20 includes counter-current traveling belt segments 28 and 30 moving in opposite directions for transporting the constituents of the particle mixture along the lengths of the electrodes 12 and 14/16.
As the only moving part, the belt 20 is a critical component of the BSS. The belt 20 moves at high speed, for example, about 40 miles an hour, in an extremely abrasive environment. The two belt segments 28, 30 move in opposite directions, parallel to centerline 18, and thus if they come into contact, the relative velocity is about 80 miles an hour. Related art belts were previously woven of abrasion resistant monofilament materials. These belts were quite expensive and lasted only about 5 hours. The mode of failure was typically longitudinal wear stripes due to longitudinal wrinkling, that would wear longitudinal holes in the belt such that it would fall apart and catch on itself. The strands would also wear where they crossed and flexed in moving through the separator. The Applicant has made attempts to improve such belts with different materials and different weaves in an attempt to find a woven material with a longer life. These attempts were unsuccessful.
Belts which are currently used in the BSS 10 are made of extruded materials which have better wear resistance than the woven belts and may last on the order of about 20 hours. The extrusion of such belts is described in commonly-owned U.S. Pat. No. 5,819,946 entitled “Separation System Belt Construction,” which is herein incorporated by reference.
Referring to FIG. 2, there is illustrated schematic drawing of a section of a belt 40 such as is currently used in the BSS of FIG. 1. Control of the geometry of the belt is desirable, but is difficult to achieve with extruded belts.
One example of the belt used in the BSS may comprise a structure formed of machine direction strands 42, i.e., strands that are disposed along a horizontal length of the belt in a direction of movement of the belt (indicated by arrow 41), and cross direction strands 46, i.e., strands that are substantially perpendicular to the machine direction strands, as illustrated in FIG. 2. The cross direction strands 46 may be made with a specific shape of a leading edge 43 of the belt. The machine direction strands 46 carry the load, i.e., a mixture of constituents, and simultaneously withstand the flexing of passing over the end rollers (see FIG. 1, 22, 24) at a rate of approximately 6 rollers per second.
The extrusion process by which belts for the BSS are currently made is necessarily a compromise of a number of factors including the choice of the polymer used, the choice of additives, the extrusion equipment, the temperatures used for the extrusion process and the extrusion rate. According to one example, the operation of the extrusion process for the current manufacture of extruded belts is as follows. A proper mix of a base polymer and additives (preferably pre-compounded together) is fed into an extrusion machine, where the mechanical action of a screw heats the material to a temperature where it is plastic, and the extrusion machine moves the plastic down a barrel and into a die. The die has a circular cross section, and has a number of grooves parallel to an axis which corresponds to the continuous machine direction strands 42. Each cross direction strand 46 is produced by moving an inner part of the die so that a circumferential groove which is filled with material empties and so forms the cross direction strand 46. Control of the geometry of the belt is mostly accomplished by adjusting the instantaneous extrusion rate during the formation of each individual cross direction strand 46. Material that ends up in the cross direction strand is not available for the machine direction strand and vice versa. It may be difficult therefore, to avoid changes in the machine direction strand cross section while changing the extrusion rate to adjust the cross strand geometry. After the web of machine direction strands and cross direction strands is formed as a circular section, it is cooled, for example, through immersion in a water bath and slit and flattened to form a flat web.
Fatigue strength is an important aspect of the belt to be used in a BSS. For good fatigue strength, stress concentrations at changes in cross section of the strand should be avoided. Maintaining uniformity of cross section is difficult however, and thus fatigue life of extruded belts is often problematic.
Conveyer belts are widely used for conveying materials, and conventional conveying belts are well developed. Usually conveyor belts are constructed of an elastomeric material with reinforcing cords of fabric. A usual practice is to use continuous solid belts without perforations. Such belts are not suitable for the present application because of the need for material to pass through the belt in the BSS.
Control of the belt geometry is also important as is described in commonly-owned U.S. Pat. No. 5,904,253, also herein incorporated by reference. Referring to FIG. 3, which is an enlarged portion of the BSS of FIG. 1, the directions of the counter-travelling belt segments 28, 30 are shown by arrows 34 and 36, respectively. As illustrated in FIG. 3, one example of a desired geometry of the belt 40, is that of an acute angle 44 on the leading edge 43 (see FIG. 2) of the cross direction strands 46.
In the current practice of extrusion, the geometry of the leading edge is controlled by adjusting the polymer composition, the additives used, and the extrusion conditions. Changing these parameters also has effects on the other properties of the belt and on its performance in the BSS. In addition, in an extrusion process, the polymers that can be used to make such belts are limited. There are a number of polymers that cannot be extruded and so are not options for belt manufacture by extrusion. In addition, large amounts of extrusion additives are needed to achieve desired belt properties through an extrusion process. However, the presence of many additives complicates the extrusion process and can pose compatibility problems, especially for food grade applications. Many of the additives needed for dimension control also act as plasticizers and increase the rate of creep and decrease wear resistance of the belt. Often changing one property in one way will have an adverse effect on other properties.
Thus known methods of manufacture of belts for BSS are subject to the limitations of the extrusion process, which limits the materials which can be used for belt construction, and compromises the geometry that can be obtained. Current belts do not have the desired long wear life, good fatigue strength, and ease of manufacture that is desired.